U.S. Pat. No. 4,917,978, issued on Apr. 17, 1990 to Ritt et al. describes a method of manufacturing a screen assembly for a CRT by the electrophotographic screening (EPS) process. The method described in the aforementioned patent includes a "fusing" step followed by a "fixing" step to increase the adherence of the phosphors to an underlying organic photoconductive (OPC) layer deposited on the interior surface of the CRT faceplate panel. In the fusing step, vapors of a solvent, such as chlorobenzene, are permitted to contact and soak the OPC layer, formed of polyvinyl carbazole, and the polymeric coupling agent that coats the phosphor materials, to render the layer and the coating tacky. Vapor soaking takes on the order of 4 to 24 hours. The panels are then dried and "fixed" by spraying multiple layers of polyvinyl alcohol (PVA) in an alcohol-water mixture onto the fused phosphors. Each spray application required about 2 to 5 minutes to achieve complete screen coverage. The "fixed" screens are then filmed either by convention spray or emulsion filming. The process described in the patent is time consuming and does not lend itself to a production environment in which the screen processing time is measured in minutes rather than hours. Additionally, it has been determined that the PVA spray applications tend to move the phosphors slightly, which might be unacceptable, depending on the amount of movement.
One method of reducing the process time is described in U.S. Pat. No. 5,028,501, issued on Jul. 2, 1991 to Ritt et al. The method eliminates the vapor soaking of the phosphor materials and the underlying OPC layer and relies, instead, on the electrostatic attraction of the triboelectrically charged phosphors particles to the OPC layer to hold the materials in position until a dry-powdered filming resin is electrostatically deposited onto the phosphor materials. The filming resin is fused by using radiant heaters which melt the dry-powdered filming resin within 1 to 5 minutes. A drawback of this latter method is that while the electrostatic deposition of the dry-powdered filming resin does not move the phosphor materials, the heating step, to melt the resin, causes some movement of the underlying phosphors. While the movement is less than that experienced using the PVA spray, it is desirable that no movement of the phosphors occur.
A method of fusing the filming resin particles in an expedient fashion to either eliminate or substantially reduce the movement of the resin particles and, thus, that of the underlying phosphor particles is described in U.S. Pat. No. 5,229,233, issued on Jul. 20, 1993 to Riddle et al. In the Riddle et al. patent, a fogging apparatus is utilized to atomize the solvent so that the filming resin is at least partially solubilized and fused with the speed of a spray, but with the gentleness of the time-consuming vapor soak described in U.S. Pat. No. 4,917,978, cited above. Nevertheless, about 2 to 3 minutes are required to completely fuse the filming resin using the fogging apparatus.
In a production facility it is desirable to secure the phosphor materials to the OPC layer in about eight seconds, or less. To this end, it is of interest to develop a process in which the phosphor materials are securely fixed to the underlying OPC layer so that movement does not occur and the materials are then filmed in an expeditious manner, or alternatively, to modify the process in such a manner that the fixing step is carried out so that it is not necessary to have a separate filming step.